\documentclass[tikz, border = 15pt]{standalone}
\begin{document}

\begin{tikzpicture}[xscale=1, yscale=1]

%\draw [help lines] (-3,-10) grid (13, 2);

% Subfigure (a)

%\draw[help lines] (-3,-2) rectangle (13,3);

\node at (5, 2.5) {\textbf{A. Fertility changes for a representative mother} };

\draw [->, ultra thick] (-2,0) -- (12,0);

\draw [thick] (0,0.5) -- (0,-0.5) node [below] {$n^o - 1$};

\draw [thick] (5,0.5) -- (5,-0.5) node [below] {$n^o$};

\draw [thick] (10,0.5) -- (10,-0.5) node [below] {$n^o+1$};

\draw [->, dashed, ultra thick] (0.7, -0.75) -- (2.5, -0.75) node [below] {\textit{Desired Increase}} -- (4.3, -0.75);

\draw [<-, dashed, ultra thick] (0.7, 0.75) -- (2.5, 0.75) node [above] {\textit{Undesired Reduction}} -- (4.3, 0.75);

\draw [->, dashed, ultra thick] (5.7, -0.75) -- (7.5, -0.75) node [below] {\textit{Undesired Increase}} -- (9.3, -0.75);

\draw [<-, dashed, ultra thick] (5.7, 0.75) -- (7.5, 0.75) node [above] {\textit{Desired Reduction}} -- (9.3, 0.75);

% Subfigure (b)

%\draw[help lines] (-3,-10) rectangle (13,-2.2);

\node at (5, -3) {\textbf{B. Fertility changes for two types of mothers} };

% Type-A
\node[align=center] at (0, -5) {\textbf{Type-A mothers} \\ $n^o_A = \bar{n}$ };

\draw [->, ultra thick] (3,-5) -- (12,-5);

\draw [thick] (5,-4.5) node [above] {$n^o_A$} -- (5,-5.5) node [below] {$\bar{n}$};


\draw [thick] (10,-4.5) -- (10,-5.5) node [below] {$\bar{n} + 1$};

\draw [->, dashed, ultra thick] (5.7, -5.75) -- (7.5, -5.75) node [below] {\textit{Undesired Increase}} -- (9.3, -5.75);

% Type-B
\node[align=center] at (0, -8) {\textbf{Type-B mothers} \\ $n^o_B = \bar{n} + 1$  };

\draw [->, ultra thick] (3,-8) -- (12,-8);

\draw [thick] (5,-7.5) -- (5,-8.5) node [below] {$\bar{n}$};

\draw [thick] (10,-7.5) node [above] {$n^o_B$} -- (10,-8.5) node [below] {$\bar{n} + 1$};

\draw [->, dashed, ultra thick] (5.7, -8.75) -- (7.5, -8.75) node [below] {\textit{Desired Increase}} -- (9.3, -8.75);

\end{tikzpicture}

\end{document}